This invention relates to a readout instrument for thermoluminescent dosimeters useful in the measurement of the dose of radiation such as gamma-ray.
A radiation dosimeter element of the thermoluninescence type emits thermoluminescence when the dosimeter element is heated to 200.degree.-400.degree. C after the exposure of the dosimeter element to radiation or radioactive ray. As is known, the intensity of the emitted thermoluminescence is proportional to the exposure dose in the preceding irradiation. Various heating methods have been employed in conventional readout instruments for thermoluminescent dosimeter elements. These heating methods are divided roughly into two types. In one type of the methods, the dosimeter element is contacted with a heat source. However, this method is rather inconvenient in practical applications, and some dosimeter elements are shaped unsuitable to this heating method. In the other type of the methods, hot air is blown against the dosimeter element. When a multi-element dosimeter is subjected to readout by the employment of a heating method of the latter type, the dosimeter is intermittently moved relatively to a nozzle for the blast of hot air so that the individual elements of the dosimeter may be heated in sequence. However, the transfer of the dosimeter elements during readout results in the consumption of a large amount of time for the overall readout operation and tends to cause appreciable errors in the readout due to a variation in the positional relationship between the nozzle and the individual dosimeter elements.
From a different point of view, there is a problem of optical noises at the readout of thermoluminescent dosimeter elements. In general, the intensity of thermoluminescence emitted from a conventional thermoluminescent dosimeter element is very feeble. It is difficult, therefore, to accomplish the readout with accuracy unless a readout instrument is designed to well suppress optical noises from various sources. Principal noise sources in the readout instrument are: (a) a dark current in the photoelectric transducer included in the instrument, (b) a leak of an external light into the instrument and (c) heat radiation from the heating section of the instrument, for example, from the walls of the heating chamber and/or certain components of the heating device such as the air nozzle and a heat exchanger. An optical noise caused by the source (a) depends primarily on the temperature of the photoelectric transducer. It is possible to suppress this noise to a satisfactorily low level by cooling the photoelectric transducer or, more conveniently, electrically compensating for this noise. A noise attributable to the source (b) can rather easily be precluded by making the readout instrument have either a fully closed construction with effective packings or a refracted construction relatively to the path of the emitted thermoluminescence.
The heat radiation (c) as the noise source is the most hard-to-solve problem in the conventional readout instruments regardless of the type of the heating method. The heating chamber and the heating device of the readout instrument always include some metal members or the like. A heated metal member, for example, emits from its surface a light of a wide wavelength range from the near infrared region to the infrared region, depending on the surface temperature of the metal member. When such a heat radiation occurs in the readout instrument, the intensity of the heat radiation governs the background level to the thermoluminescence. Since the intensity of the described heat radiation is variable with a change in the temperature, the occurrence of the heat radiation inevitably results in an irregular fluctuation of the background level and obstructs the measurement of the feeble thermoluminescence.